Fiber optic communication generally involves modulating optical signals at high bit rates and transmitting the modulated optical signals over optical fibers. For example, in a wavelength division multiplexed (WDM) fiber optic communications system, optical carrier signals at a sequence of distinct wavelengths are separately modulated by information channels and then multiplexed onto a single optical fiber. Efforts continue toward increasing the data capacity of fiber optic communications systems.
One of the key features of a wavelength division multiplexed (WDM) system is spectral efficiency, which determines how many bits/sec of data can be transmitted per unit of available bandwidth. The available bandwidth in practical WDM systems is limited by optical amplifiers which are used to periodically boost the optical power for a given transmission length. However, upgrading the capacity of existing optical networks by replacing in-ground fiber and amplifiers is extremely costly. Various techniques have been proposed to enhance the spectral efficiency of WDM systems to improve spectral efficiency (i.e., bits per second transmitted per unit of available bandwidth) of the existing optical infrastructure. That is, because of the cost associated with upgrading or replacing the available WDM bandwidth, various techniques have been proposed to enhance the spectral efficiency of WDM systems to approach or exceed 1 bit/sec/Hz, which is the theoretical limit for a simple binary modulation format.
However, most of these techniques involve either complex binary modulation formats to limit the channel bandwidth for a given data rate, or an attempt to take advantage of multi-level modulation formats and polarization division multiplexing to increase the spectral efficiency. However, many of these complex techniques require costly transmitters and receivers with specialized, expensive electronics.